Glass that is fairly clear in color and highly transmissive to visible light (e.g., at least 75% transmissive, or even more preferably at least 80% transmissive) is sometimes desirable. One way of achieving such as glass is to use very pure base glass materials (e.g., substantially free of colorants such as iron). However, base materials with a high degree of purity are expensive and thus not always desirable and/or convenient. In other words, for example, the removal of all iron from glass raw materials has certain practical and/or economical limits.
First and second standard high transmission glass compositions are as follows. These standard compositions, and the methods of making the same, will be referred to herein as Comparative Example (CE) 1 and Comparative Example (CE) 2. The amounts of the various materials below for CE 1 and CE 2 are listed in terms of weight percentage (%) in the glass, unless otherwise indicated, and the optical characteristics were measured at a reference thickness of about 6 mm. Visible transmission was measured in terms of Lta (Ill. C. 2 deg.), whereas the a* and b* transmissive color values were measured in accordance with Ill. D65, 10 deg.
COMPARATIVE EXAMPLES 1 AND 2Element/CharacteristicCE 1CE 2SiO272.8971.9Na2O13.4813.66CaO8.579.12MgO4.063.76Al2O30.161.02K2O0.080.24SO30.2230.186TiO20.010.04Cr2O38 ppm5 ppmFe2O3 (total iron)0.1120.099% FeO0.0230.029Glass Redox0.210.26Batch Redox6.76.3Lta (Tvis)(%)89.189.06a*−1.48−1.43b*0.380.12
As can be appreciated from the above, the aforesaid Comparative Example (CE) glasses include rather low amounts of total iron. The total amount of iron present is expressed herein in terms of Fe2O3 in accordance with standard practice. However, typically, not all iron is in the form of Fe2O3. Instead, iron is usually present in both the ferrous state (Fe2+; expressed herein as FeO, even though all iron in the glass may not be in the form of FeO) and the ferric state (Fe3+). Iron in the ferrous state (Fe2+; FeO) is a blue-green colorant, while iron in the ferric state (Fe3+) is a yellow-green colorant. The blue-green colorant of ferrous iron (Fe2+; FeO) is of particular concern when seeking to achieve a fairly clear or neutral colored glass, since as a strong colorant it introduces significant color into the glass. While iron in the ferric state (Fe3+) is also a colorant, it is of less concern when seeking to achieve a glass fairly clear in color since iron in the ferric state tends to be weaker as a colorant than its ferrous state counterpart.
When making high transmission glasses, it has been found that the use of elemental carbon (C) (introduced as coke, carbocite, anthracite, or the like) as a reducing agent needed for sulfate refining is undesirable because it tends to cause substantial formation of ferrous iron (Fe2+; FeO). As explained above, ferrous iron is of particular concern when trying to achieve highly transparent clear glass because ferrous iron results in both a loss of transmittance and the appearance of blue-green coloration in the glass.
In view of the above, it is apparent that there exists a need in the art for a new technique for making highly transparent substantially clear glasses which can reduce the formation of ferrous iron, thereby permitting high transmission and substantially clear color to be realized.